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Date:   Mon, 9 Nov 2020 16:44:10 +0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Filipe Manana <fdmanana@suse.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, peterz@infradead.org,
        Jan Kara <jack@suse.cz>, David Sterba <dsterba@suse.com>,
        Nikolay Borisov <nborisov@suse.com>, Qu Wenruo <wqu@suse.com>
Subject: Re: possible lockdep regression introduced by 4d004099a668
 ("lockdep: Fix lockdep recursion")
Message-ID: <20201109084410.GA286534@boqun-archlinux>
References: <a5cf643b-842f-7a60-73c7-85d738a9276f@suse.com>
 <20201103140828.GA2713762@boqun-archlinux>
 <283e8f43-91d1-7a19-ed9a-ac6cd13d41ce@suse.com>
 <20201104022236.GA1118860@boqun-archlinux>
 <20201104033914.GA2739173@boqun-archlinux>
 <f8a30ef7-f03b-26d5-21df-f7f8ac373bae@suse.com>
 <7fd90864-a15d-8757-017e-7d73a7d4eac8@suse.com>
 <20201105011012.GB2748545@boqun-archlinux>
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Hi Filipe,

On Thu, Nov 05, 2020 at 09:10:12AM +0800, Boqun Feng wrote:
> On Wed, Nov 04, 2020 at 07:54:40PM +0000, Filipe Manana wrote:
> [...]
> > 
> > Ok, so I ran 5.10-rc2 plus your two patches (the fix and the debug one):
> > 
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index b71ad8d9f1c9..b31d4ad482c7 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -539,8 +539,10 @@ static struct lock_trace *save_trace(void)
> >                 LOCK_TRACE_SIZE_IN_LONGS;
> > 
> >         if (max_entries <= 0) {
> > -               if (!debug_locks_off_graph_unlock())
> > +               if (!debug_locks_off_graph_unlock()) {
> > +                       WARN_ON_ONCE(1);
> >                         return NULL;
> > +               }
> > 
> >                 print_lockdep_off("BUG: MAX_STACK_TRACE_ENTRIES too low!");
> >                 dump_stack();
> > @@ -5465,7 +5467,7 @@ noinstr int lock_is_held_type(const struct
> > lockdep_map *lock, int read)
> >         unsigned long flags;
> >         int ret = 0;
> > 
> > -       if (unlikely(!lockdep_enabled()))
> > +       if (unlikely(debug_locks && !lockdep_enabled()))
> >                 return 1; /* avoid false negative lockdep_assert_held() */
> > 
> >         raw_local_irq_save(flags);
> > 
> > With 3 runs of all fstests, the WARN_ON_ONCE(1) wasn't triggered.
> > Unexpected, right?
> > 
> 
> Kinda, that means we still don't know why lockdep was turned off.
> 
> > Should I try something else?
> > 
> 
> Thanks for trying this. Let me set up the reproducer on my side, and see
> if I could get more information.
> 

I could hit this with btrfs/187, and when we hit it, lockdep will report
the deadlock and turn off, and I think this is the root cause for your
hitting the original problem, I will add some analysis after the lockdep
splat.

[12295.973309] ============================================
[12295.974770] WARNING: possible recursive locking detected
[12295.974770] 5.10.0-rc2-btrfs-next-71 #20 Not tainted
[12295.974770] --------------------------------------------
[12295.974770] zsh/701247 is trying to acquire lock:
[12295.974770] ffff92cef43480b8 (&eb->lock){++++}-{2:2}, at: btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
[12295.974770] 
               but task is already holding lock:
[12295.974770] ffff92cef434a038 (&eb->lock){++++}-{2:2}, at: btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
[12295.974770] 
               other info that might help us debug this:
[12295.974770]  Possible unsafe locking scenario:

[12295.974770]        CPU0
[12295.974770]        ----
[12295.974770]   lock(&eb->lock);
[12295.974770]   lock(&eb->lock);
[12295.974770] 
                *** DEADLOCK ***

[12295.974770]  May be due to missing lock nesting notation

[12295.974770] 2 locks held by zsh/701247:
[12295.974770]  #0: ffff92cef3d315b0 (&sig->cred_guard_mutex){+.+.}-{3:3}, at: bprm_execve+0xaa/0x920
[12295.974770]  #1: ffff92cef434a038 (&eb->lock){++++}-{2:2}, at: btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
[12295.974770] 
               stack backtrace:
[12295.974770] CPU: 6 PID: 701247 Comm: zsh Not tainted 5.10.0-rc2-btrfs-next-71 #20
[12295.974770] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v4.0 12/17/2019
[12295.974770] Call Trace:
[12295.974770]  dump_stack+0x8b/0xb0
[12295.974770]  __lock_acquire.cold+0x175/0x2e9
[12295.974770]  lock_acquire+0x15b/0x490
[12295.974770]  ? btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
[12295.974770]  ? read_block_for_search+0xf4/0x350 [btrfs]
[12295.974770]  _raw_read_lock+0x40/0xa0
[12295.974770]  ? btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
[12295.974770]  btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
[12295.974770]  btrfs_search_slot+0x6ac/0xca0 [btrfs]
[12295.974770]  btrfs_lookup_xattr+0x7d/0xd0 [btrfs]
[12295.974770]  btrfs_getxattr+0x67/0x130 [btrfs]
[12295.974770]  __vfs_getxattr+0x53/0x70
[12295.974770]  get_vfs_caps_from_disk+0x68/0x1a0
[12295.974770]  ? sched_clock_cpu+0x114/0x180
[12295.974770]  cap_bprm_creds_from_file+0x181/0x6c0
[12295.974770]  security_bprm_creds_from_file+0x2a/0x40
[12295.974770]  begin_new_exec+0xf4/0xc40
[12295.974770]  ? load_elf_phdrs+0x6b/0xb0
[12295.974770]  load_elf_binary+0x66b/0x1620
[12295.974770]  ? read_hv_sched_clock_tsc+0x5/0x20
[12295.974770]  ? sched_clock+0x5/0x10
[12295.974770]  ? sched_clock_local+0x12/0x80
[12295.974770]  ? sched_clock_cpu+0x114/0x180
[12295.974770]  bprm_execve+0x3ce/0x920
[12295.974770]  do_execveat_common+0x1b0/0x1f0
[12295.974770]  __x64_sys_execve+0x39/0x50
[12295.974770]  do_syscall_64+0x33/0x80
[12295.974770]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[12295.974770] RIP: 0033:0x7f6aaefc13cb
[12295.974770] Code: 48 3d 00 f0 ff ff 76 e7 f7 d8 64 41 89 00 eb df 0f 1f 80 00 00 00 00 f7 d8 64 41 89 00 eb dc f3 0f 1e fa b8 3b 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 75 4a 0f 00 f7 d8 64 89 01 48
[12295.974770] RSP: 002b:00007ffd33b54d58 EFLAGS: 00000207 ORIG_RAX: 000000000000003b
[12295.974770] RAX: ffffffffffffffda RBX: 00007f6aaf28bc88 RCX: 00007f6aaefc13cb
[12295.974770] RDX: 00007ffd33b5fd98 RSI: 00007f6aaf28bc88 RDI: 00007ffd33b55280
[12295.974770] RBP: 00007ffd33b54d80 R08: 00007ffd33b54ce0 R09: 0000000000000001
[12295.974770] R10: 0000000000000008 R11: 0000000000000207 R12: 00007ffd33b55280

The deadlock senario reported by this splat is as follow:

	CPU 0					CPU 2
	=====					=====
	btrfs_search_slot():
	  btrfs_tree_read_lock_atomic():
	    read_lock(&eb->lock);		<a random writer>
						write_lock(&eb->lock);
						// waiting for the lock
	  ...
	  btrfs_tree_read_lock_atomic():
	    read_lock(&eb->lock); // blocked because the fairness.

In short, you can not use nested read_lock() on the same lock. However,
I'm not sure whether we have the real problem here, because I don't
think btrfs_search_slot() can pick the same extent buffer twice in
btrfs_search_slot(). Also copy the author of the code for more
information.

Regards,
Boqun

> Regards,
> Boqun
> 
> > Thanks.
> > 
> >